A conventional power steering device of this type is adapted to transmit drive force of a motor through a reduction mechanism and a transmission mechanism such as gear or belt to a steering shaft or a rack shaft.
The conventional power steering device as mentioned above is of the type in which a vehicle speed is detected by a vehicle speed sensor so that the assist is removed when the detected speed reaches or exceeds a constant value. However, since the speed signal from the speed sensor is supplied as a series of pulses the number of which is proportional to the vehicle speed, a construction of an input circuit and a program for calculating the vehicle speed become complicated, resulting in high costs of a microcomputer and a vehicle speed input circuit to be used in a control unit.
This invention was made in view of such problems as mentioned above and an object thereof is to obtain a motor driven power steering control device which can use an inexpensive microcomputer by simplifying constructions of the input circuit and the control program.